Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of hardfacing materials used to improve the wear resistance of metals. More particularly, the invention relates to compositions of hardfacing materials which are particularly suitable for use on drill bits.
2. Background Art
Bits for drilling oil wells (xe2x80x9crock bitsxe2x80x9d) typically have steel bodies (xe2x80x9cbit bodiesxe2x80x9d) that are connected at the bottom of drill strings. One or more roller cones are rotatably mounted to the bit body. These roller cones have a plurality of teeth attached thereto. These teeth crush, gouge, and scrape rock at the bottom of a hole being drilled. Several types of roller cone drill bits are available for drilling wellbores through earth formations, including insert bits (e.g. tungsten carbide insert bit, TCI) and xe2x80x9cmilled toothxe2x80x9d bits. The following illustration and discussion will use the milled tooth bits as examples. However, it should be noted that the invention is not limited to this type of bits. Instead, the invention is applicable to any rock bit.
Milled tooth bits include one or more roller cones rotatably mounted to a bit body. The one or more roller cones are typically made from steel and include a plurality of teeth formed integrally with the material from which the roller cones are made. Typically, a hardfacing material is applied, such as by arc or gas welding, to the exterior surface of the teeth to improve the wear resistance of the teeth. The hardfacing material typically includes one or more metal carbides, which are bonded to the steel teeth by a metal alloy (xe2x80x9cbinder alloyxe2x80x9d). In effect, the carbide particles are suspended in a matrix of metal forming a layer on the surface. The carbide particles give the hardfacing material hardness and wear resistance, while the matrix metal provides fracture toughness to the hardfacing.
Many factors affect the durability of a hardfacing composition in a particular application. These factors include the chemical composition and physical structure (size and shape) of the carbides, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy.
The metal carbide most commonly used in hardfacing is tungsten carbide. Small amounts of tantalum carbide and titanium carbide may also be present in such material, although these other carbides are considered to be deleterious. It is quite common to refer to the material in the hardfacing merely as xe2x80x9ccarbidexe2x80x9d without characterizing it as tungsten carbide. It should be understood that as used herein, xe2x80x9ccarbidexe2x80x9d generally means tungsten carbide.
Many different types of tungsten carbides are known based on their different chemical compositions and physical structure. Three types of tungsten carbide commonly employed in hardfacing drill bits are: cast tungsten carbide, macro-crystalline tungsten carbide, and cemented tungsten carbide (also known as sintered tungsten carbide). The most common among these is possibly crushed cast carbide.
Tungsten forms two carbides, monotungsten carbide (WC) and ditungsten carbide (W2C). Tungsten carbide may also exist as a mixture of these two forms with any proportion between the two. Cast carbide is a eutectic mixture of the WC and W2C compounds, and as such the carbon content in cast carbide is sub-stoichiometric, i.e., it has less carbon than the more desirable WC form of tungsten carbide. Cast carbide is prepared by freezing carbide from a molten state and crushing and comminuting the resultant particles to the desired particle size.
Macro-crystalline tungsten carbide is essentially stoichiometric WC in the form of single crystals. While most of the macro-crystalline tungsten carbide is in the form of single crystals, some bicrystals of WC are found in larger particles. Macro-crystalline WC is a desirable hardfacing material because of its toughness and stability.
The third type of tungsten carbide used in hardfacing is cemented tungsten carbide, also known as sintered tungsten carbide. Cemented tungsten carbide comprises small particles of tungsten carbide (e.g., 1 to 15 microns) bonded together with cobalt. Cemented tungsten carbide is made by mixing organic wax, tungsten carbide and cobalt powders, pressing the mixed powders to form a green compact, and xe2x80x9csinteringxe2x80x9d the composite at temperatures near the melting point of cobalt. The resulting dense cemented carbide can then be crushed and comminuted to form particles of cemented tungsten carbide for use in hardfacing.
In addition to these three types of commonly used carbides, carburized tungsten carbide may also be used to provide desired property. Other compositions for hardfacing are disclosed, for example in U.S. Pat. No. 4,836,307 issued to Keshavan et al., and U.S. Pat. No. RE 37,127 issued to Schader et al.
As mentioned above, conventional hardfacing usually comprises particles of tungsten carbide bonded to the steel teeth by a metal alloy. In effect, the carbide particles are suspended in a matrix of metal forming a layer on the surface. Most hardfacing on rock bits employs steel as the matrix, although other alloys may also be used. Such steel or other alloys will be generally referred to as a binder alloy. Hardfacing compositions are typically applied from tube rods, for example as disclosed in U.S. Pat. No. 5,250,355 issued to Newman et al.
A typical technique for applying hardfacing to the teeth on a rock bit is by oxyacetylene or atomic hydrogen welding. A welding xe2x80x9crodxe2x80x9d or stick is typically formed of a tube of mild steel sheet enclosing a filler which mainly comprises carbide particles. The filler may also include deoxidizer for the steel, flux and a resin binder. The hardfacing is applied by melting an end of the rod on the face of the tooth. The steel tube melts to weld to the steel tooth and provide the matrix for the carbide particles. The deoxidizer alloys with the mild steel of the tube.
Although mild steel sheet is used when forming the tubes, the steel in the hardfacing on a finished a rock bit is a hard, wear resistant alloy steel. The conversion from a mild steel to the hard, wear resistant alloy steel occurs when the deoxidizers (which contain silicon and manganese) in the filler and tungsten, carbon, and possibly cobalt, from the tungsten carbide dissolve and mix with the steel during welding. There may also be some mixing with alloy steel from the teeth on the cone.
Advances in wear resistance of hardfacing are desirable to enhance the footage a drill bit can drill before becoming dull, and to enhance the rate of penetration of such drill bits. Such improvements translate directly into reduction of drilling expense. The composition of a hardfacing material, and the physical structure of the hardfacing material applied to the surfaces of a drill bit are related to the degree of wear resistance and toughness. It is desirable to have a composition of hardfacing material, which when applied to wear surfaces, provides improved wear resistance and toughness.
One aspect of the invention relates to a hardfacing composition for a drill bit, which includes a carbide phase comprising from about 80% to less than 100% by weight of a combination of sintered carbide pellets and crushed cast carbide and from more than 0% to about 20% by weight of carburized tungsten carbide. The composition includes a binder alloy.
Another aspect of the invention relates to a hardfacing composition for a drill bit, which includes a carbide phase comprising from about 80% to less than 100% by weight of a combination of sintered carbide pellets and crushed cast carbide and from more than 0% to about 20% by weight of carburized tungsten carbide. The sintered carbide pellets in this composition includes more than one population of pellets having different size ranges. The composition includes a binder alloy.
Another aspect of the invention relates to a hardfacing composition for a drill bit, which includes a carbide phase comprising from about 80% to about 95% by weight of a combination of sintered carbide pellets and crushed cast carbide and from about 5% to about 20% by weight of macro-crystalline tungsten carbide. Such compositions include a binder alloy.
Yet another aspect of the invention relates to roller cone drill bits. According to one embodiment of the invention, a roller cone drill bit includes a bit body and at least one roller cone rotatably mounted to the bit body. The at least one roller cone includes at least one cutting element. The cutting element has on its exterior surface a hardfacing. The hardfacing on the cutting element was formed with a hardfacing composition that includes a binder alloy and a carbide phase, which includes from about 80% to less than 100% by weight of a combination of sintered carbide pellets and crushed cast carbide, and from more than 0% to about 20% by weight of carburized tungsten carbide.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.